FIGS. 12A and 12B show a conventional small antenna used in a mobile communication device such as a pager. Loop antenna 100 made of conductive metal is disposed at a side of display 105 disposed on base substrate 104. Antenna 100 has an opening directly perpendicular to the base substrate. The opening of this antenna is perpendicular to a human body when the pager is used in the vicinity of the human body. Since the human body can be treated as a reflector, a magnetic current generated within the human body has the same direction as a magnetic dipole formed by loop antenna 100. Accordingly, antenna gain can be increased because magnetic fields are added at the front of the human body.
One end of loop antenna 100 is DC-short-circuited to feeding part 102 via first matching capacitor 101a, while the other end thereof is DC-short-circuited to ground short-circuiting part 103 via third matching capacitor 101c. Feeding part 102 is coupled to ground short-circuiting part 103 via second matching capacitor 101b. The element length of the loop antenna is basically set to be equal to one-half wavelength of an operating frequency. frequency. For example, pagers in Japan use a 280 MHz band, and one-half of the wavelength of the frequency is thus about 500 mm. However, an antenna having an element length of 500 mm is impractical to build into a small pager. Thus, the loop antenna has its size changed by having the element length shorter than 500 mm for storage in the pager, and the antenna is matched with matching capacitors 101a, 101b, and 10c. 
A conventional antenna described above requires the capacitors for impedance matching, and power loss in the capacitors causes considerable degradation of the radiation gain of the antenna. In addition, a ground pattern and a component which are mounted on the substrate cause the radiation gain of the antenna to degrade.
FIG. 13 shows another conventional small antenna used in a mobile communication device, such as the pager. This antenna is disclosed in Japanese Patent Publication No. 6-93635. Metal plate 1101 is a ground plane, which is an element of a microstrip antenna. Printed board 1105 has a circuit for radio communication mounted on metal plate 1101, and conductive plate 1102 is placed over metal plate 1101 via dielectric member 1104. Conductive plate 1102 has a smaller width than metal plate 1101 and faces metal plate 1101. A clearance between metal plate 1101 and conductive plate 1102 is filled with dielectric member 1104. Printed board 1105 is mounted so as not to cover the part where metal plate 1101 and conductive plate 1102 face each other. Metal plate 1101 and conductive plate 1102 are mechanically and electrically connected to each other at their respective ends with connecting plate 1103, so that metal plate 1101, conductive plate 1102, and connecting plate 1103 cooperatively form a U-shaped microstrip antenna. In order to tune this microstrip antenna to a desired frequency, the other end of conductive plate 1102 is grounded via capacitor 1106, and feeder 1107 is adjusted for matching.
The above-described microstrip antenna, due to the inclusion of the conductive plate, requires accurate metalworking of its dimensions for mass production. Thus, it is difficult to mount the conductive plate to the metal plate. Moreover, single capacitor 1106, since providing the microstrip antenna with a small range of adjustable impedance, may not achieve the impedance matching due to the effect of a component or metal placed in the vicinity of the microstrip antenna. Further, the antenna, since being adaptable to only one frequency band, cannot change operating frequency according to the application.